1. Technical Field
This disclosure relates to aircraft flight control system and hydraulic system and, more particularly, to a method and apparatus for maintaining control of the aircraft if the primary hydraulic system is compromised.
2. Description of the Related Art
Flight control systems in commercial aircraft have redundancy to permit control of the aircraft in the event of failure of part of the system. For the hydraulic actuators that control the flight control surfaces, backup systems are present on each aircraft so that if one of the hydraulic systems fails, other systems are available to provide power to control sufficient flight control surfaces.
A commercial aircraft typically has a plurality of independent central hydraulic systems, usually two or three, depending on the type of aircraft. The hydraulic pressure in each central hydraulic system is generated by one or more centrally located hydraulic pumps which are driven directly or indirectly by a main power source such as an engine. Each central hydraulic system has a plurality of aircraft systems which draw hydraulic power from it to actuate components in the airplane, and the flight control system is one such system. High pressured hydraulic fluid in each central hydraulic system is carried in hydraulic lines to the hydraulic actuators at each flight control surface. Servo valve at each hydraulic actuator controls the application of pressurized fluid to the hydraulic actuators. The servo valves operate based on electrical signals transmitted on electric wires throughout the aircraft, thus providing a fly-by-wire control system.
Among the methods to provide redundancy of power is to have the flight control of hydraulic actuators powered by different independent hydraulic systems. In addition, on a flight control surface whose continued operation is critical, there may be multiple actuators, each drawing power from a different hydraulic system. The locations of the hydraulic actuators that receive pressure from each central hydraulic system are selected such that sufficient flight control surfaces are available to support continued safe flight and landing following the failure of any two hydraulic systems.
Because each central hydraulic system consists of large pumps and tubing that extends through the entire aircraft to each flight control surface, there are significant economical and performance advantages to being able to reduce the number of required systems, especially with large aircraft. At the same time, an equivalent or better level of safety must be assured for the aircraft. In order to realize this, two types of actuators, EHA and EBHA, have been proposed in the prior art.
An Electro-hydrostatic actuator (EHA) is an electric actuator which uses the central electrical system to power the motor which is connected to a two-way pump, both of which are typically mounted on the actuator. A hydraulic reservoir and lines are provided at each actuator, and therefore, it requires no connection to a central hydraulic system. During normal operation, the outputs of the two-way pump are connected directly to the actuator such that the movement of the pump translates directly to the extension and retraction movements of the piston/rod assembly. The control signals for the motor and pump are carried on control wires that go throughout the aircraft. By replacing all of the hydraulic actuators on a particular hydraulic system with EHAs and routing electrical power cables to each, it becomes possible to eliminate that hydraulic system.
There are two major disadvantages associated with EHAs and the system comprising them. One is the reduced reliability of each local hydraulic system comprising a motor, motor driver, and motor driver electronics. They are subject to being overworked and are more likely to fail than a central hydraulic system or hydraulic actuator or such a central system. Since they are required at each actuator, failure of one of these components will result in failure of the EHA.
The second problem is force fight, which will now be explained in more detail. In order to reduce the number of required hydraulic systems, some of the conventional hydraulic actuators may be replaced with EHAs. In many cases, it is desirable to replace one of the actuators on a particular surface, while leaving the other one hydraulic. When multiple actuators on a particular surface are operating simultaneously, it is called an active-active system, or the actuators are referred to as working in an active-active fashion. In an active-active system, even minor differences in the timing of operation of the valves, pumps, and pressure in each system creates a force fight. When dissimilar actuators, such as EHA and conventional hydraulic actuator, are used on a surface in an active-active fashion, blending them to work in perfect unison is very difficult, and so, a force fight is very likely to occur in this situation. If there is a substantial force fight, the electric motor, pump, actuator components, or surrounding structure may be overloaded and subject to premature failure.
In order to overcome the difficulties associate with reduced reliability and force fight, one current solution is to operate only one actuator on a surface at a time, keeping the EHA on standby until the hydraulic actuator on the same surface fails. Once the hydraulic actuator fails, the electric motor and pump in the EHA are activated to maintain control of the surface. This is called an active-standby system since one actuator is on standby and is not active until the other actuator fails. This circumvents the reliability issue of the EHA because the EHA is used only after failure. It also avoids force fight by activating only one actuator on any surface at once. While an active-standby system offers some solutions to these difficulties, there are many other advantages to an active-active system that make it more attractive.
An electric backup hydraulic actuator (EBHA) is a hybrid actuator employing both electric and hydraulic powers, and it is another prior art system that may be used to allow reduction in the central hydraulic system. It is a combination of an EHA and conventional hydraulic actuator, and it has connections to both the central hydraulic system and the electrical system. In an EBHA, the primary source of power is provided by hydraulic fluids lines of a particular hydraulic system, as is standard. In addition, EBHA also has a local electric motor and two-way pump, and in the event of failure of the central hydraulic system, the local electric motor and pump are switched on by electric signals on the distributed control line to power and control the actuator in the same way as an EHA. Although it requires connection to the hydraulic system, because it remains functional following the complete failure of the hydraulic system, by connecting EBHAs to appropriate flight control surfaces, continued safe flight and landing is possible following a complete loss of the central hydraulic systems, which might occur if there are only two hydraulic systems.
EBHA overcomes the reduced reliability problem of EHAs by using the low-reliability components only as backup and activating them only following the failure of the primary power or control components. On the other hand, the force fight problem between dissimilar actuators on a particular surface used in an active-active fashion still remains. If one EBHA and one hydraulic actuator is coupled to a surface as a pair, for example, the EBHA functions as a hydraulic actuator during normal operation, because the electric motor is turned off, and so, there is no increase in the level of force fight as compared to having two hydraulic actuators. Following the failure of the primary hydraulic source, however, the EBHA behaves as an EHA, and so, it is subject to the same aforementioned force fight issue associated with having an EHA and a hydraulic actuator on the same surface and having them work in an active-active fashion. In addition, coupling only EBHAs to a surface and having them work in an active-active fashion would also result in the same situation when one of the primary hydraulic power sources fails.
Some systems of the prior art are shown in U.S. Pat. Nos. 5,181,380, 6,625,982, 4,472,780 and 5,493,497.
Thus, each of the systems comprising EHAs or EBHAs, while offering some potential advantages over the conventional flight control systems comprising all hydraulic actuators, have significant difficulties being applied particularly to active-active systems, which in itself has advantages over active-standby systems. An improved system and actuator would provide significant advantages for aircraft operation, especially if it resulted in reduced overall cost and weight at the same time as providing increased reliability and increased safety.